The present invention concerns a method for the production of friction linings, in particular clutch linings, using fiber material, heat-hardenable binding agents and customary additives, shaping the mass containing such components, strengthening under heat and pressure, and curing, as well as friction linings themselves, in particular clutch facings, not only containing asbestos fibers, but also substantially free of asbestos fibers, which contain the hardened binding agents and customary additives.
Friction linings, not only brakes linings (which thus serve for the dissipation of energy), but also clutch in machine and engine construction, in particular to a great facings (which serve for the transfer of energy) are used extent within the scope of the motor vehicle industry. They mechanical strength, the friction behavior, the thermal capacity are provided for different demands, with regard to the level, the insensitivity with regard to oil, water and corroding influences, the climatic insensitivity and the like.
Friction linings, in particular clutch facings, such as clutch plates and the like, are generally constructed on the basis of yarns and/or webs, in particular of asbestos yarn and asbestos webs. The yarn or web is impregnated or treated with binding agents and aggregates, frequently with a mixture of these materials. The binding agents involved are mostly heat-hardenable phenol or cresol resins, and partly also naturally or synthetically produced lattices with the necessary vulcanization aids, catalysts, among others. Included in the customary aggregates are substances which influence the coefficient of friction and the wear behavior, organic or inorganic filler materials, such as pearl white (artificial barium sulfate), kaolin, graphite, metal oxides, asbestos powder, oils and polymerisates.
Various types of processes are known for the production of the friction linings. For example, one can so proceed that the yarn or web is brought together with a dry mixture of binding agent and aggregate, then put into suitable form and strengthened and hardened under heating and pressure. The hot extrusion temperature lies mostly within the range from 150.degree. to 200.degree. C., while the amount of pressure applied is usually in the range from about 17 to 25 N/mm.sup.2. The pressing time is dependent upon the thickness of the friction lining, and can amount to several minutes. The hardening follows frequently at higher temperatures over a longer period of time, often up to several hours. Instead of the addition of binding agents and aggregates in solid form, it frequently follows through impregnation of the yarn or the web with a solution of heat-hardenable binding agent in an organic solvent. The customary additives and possibly adjuvants can be brought in separately or together with the solution of the binding agent, if necessary suspended therein.
With all of these known methods there are notable shortcomings. Thus, the dry method, on account of the handling of finely divided, solid materials, and the therewith connected danger of dust, is very problematical on occupational hygiene grounds. This is so in particular when the dry mixture contains fine dust-like portions of mineral fiber materials. The use of solvents indeed overcomes the danger of dust but has, however, important disadvantages of its own. On hygienic and technical safety grounds, one can not simply allow the solvent to evaporate. On account of the possible danger of explosion, and also on economical grounds, the driving off of the solvent must follow in a closed apparatus. The solvent can indeed then be condensed and subjected to a distillation for further use. Pertaining to apparatus, this manner of operation is nevertheless quite expensive, anticipating unavoidable loss of solvent. There thus still occurs ecological problems.
Attempts to use, instead of yarns and webs, aqueous fiber suspensions, in which the binding agent and other aggregates are mixed in, have likewise turned out to be unsatisfactory. The waste water resulting after passing the aqueous slurry through screens still constituted a considerable environmental burden, in particular on account of its content of phenol or cresol compounds, and could not be drained away without reworking. Apart from this, an unsatisfactory loss of binding agent and aggregates must actually be put up with.